T. de Haas

Earth-like aqueous debris-flow activity on Mars at high orbital obliquity in the last million years

Liquid water is currently extremely rare on Mars, but was more abundant during periods of high obliquity in the last few millions of years. This is testified by the widespread occurrence of mid-latitude gullies: small catchment-fan systems. However, there are no direct estimates of the amount and frequency of liquid water generation during these periods. Here we determine debris-flow size, frequency and associated water volumes in Istok crater, and show that debris flows occurred at Earth-like frequencies during high-obliquity periods in the last million years on Mars. Results further imply that local accumulations of snow/ice within gullies were much more voluminous than currently predicted; melting must have yielded centimetres of liquid water in catchments; and recent aqueous activity in some mid-latitude craters was much more frequent than previously anticipated.

Topographic forcing of tidal sandbar patterns for irregular estuary planforms

Estuaries typically show converging planforms from the sea into the land. Nevertheless, their planform is rarely perfectly exponential and often shows curvature and presence of embayments. Here we test the degree to which the shapes and dimensions of tidal sand bars depend on estuary planform. We assembled a dataset with 35 estuary planforms and properties of 190 tidal bars to induce broad-brush but significant empirical relations between channel planform, hydraulic geometry and bar pattern, and test a linear stability theory for bar pattern. We found that the location where bars form is largely controlled by the excess width of a channel, which is calculated as the observed channel width minus the width of an ideal exponentially widening estuary. In general, the summed width of bars approximates the excess width as measured in the along-channel variation of three estuaries for which bathymetry was available as well as for the local measurements in the 35 investigated estuaries. Bar dimensions can be predicted by either the channel width or the tidal prism, because channel width also strongly depends on local tidal prism. Also braiding index was predicted within a factor 2 from excess width divided by the predicted bar width. Our results imply that estuary planform shape, including mud flats and salt marsh, as well as bar pattern depend on inherited Holocene topography and lithology and that eventually convergent channels will form if sufficient sediment is available.

Time will tell: temporal evolution of Martian gullies and palaeoclimatic implications.

Abstract To understand Martian palaeoclimatic conditions and the role of volatiles therein, the spatiotemporal evolution of gullies must be deciphered. While the spatial distribution of gullies has been extensively studied, their temporal evolution is poorly understood. We show that gully size is similar in very young and old craters. Gullies on the walls of very young impact craters (less than a few myr) typically cut into bedrock and are free of latitude-dependent mantle (LDM) and glacial deposits, while such deposits become increasingly evident in older craters. These observations suggest that gullies go through obliquity-driven degradation–accumulation cycles over time, controlled by: (1) LDM emplacement and degradation; and (2) glacial emplacement and removal. In glacially-influenced craters, the distribution of gullies on crater walls coincides with the extent of glacial deposits, which suggests that the melting of snow and ice played a role in the formation of these gullies. Yet, present-day activity is observed in some gullies on formerly glaciated crater walls. Moreover, in very young craters, extensive gullies have formed in the absence of LDM and glacial deposits, showing that gully formation can also be unrelated to these deposits. The Martian climate varied substantially over time, and the gully-forming mechanisms are likely to have varied accordingly.

Effects of debris-flow magnitude-frequency distribution on avulsions and fan development: Effects of debris-flow magnitude-frequency distribution on avulsions and fan development

Shifts in the active channel on a debris‐flow fan, termed avulsions, pose a large threat because new channels can bypass mitigation measures and cause damage to settlements and infrastructure. Recent, but limited, field evidence suggests that avulsion processes and tendency may depend on the flow‐size distribution, which is difficult to constrain in the field. Here, we investigate how the flow magnitude‐frequency distribution and the associated flow‐magnitude sequences affect avulsion on debris‐flow fans. We created three experimental fans with contrasting flow‐size distributions: (1) a uniform distribution, (2) a steep double‐Pareto distribution with many flows around the mean and a limited number of large flows, and (3) a shallow double‐Pareto distribution with fewer flows around the mean and more abundant large flows. The fan formed by uniform flows developed through regular sequences of stepwise channelization, backstepping of deposition toward the fan apex, and avulsion over multiple flows. In contrast, the wide range of sizes in the double‐Pareto distributions led to distinct avulsion mechanisms and fan evolution. Here, large flows could overtop channels, creating levee breaches that could initiate avulsion immediately or in subsequent events. Moreover, sequences of small‐ to moderately‐sized flows could deposit channel plugs, triggering avulsion in the next large flow. This mechanism was most common on the fan formed by a steep double‐Pareto distribution but was rare on the fan formed by a shallow double‐Pareto distribution, where large flows were more frequent. We infer that some flow‐size distributions are more likely to cause avulsions ‐ especially those that produce abundant sequences of small flows followed by a large flow. Critically, avulsions in our experiments could occur by either large single events or over multiple flows. This observation has important implications for hazard assessment on debris‐flow fans, suggesting that attention should be paid to flowhistory as well as flow size.